The present invention relates generally to steel well casing, and more specifically to new and useful improvements in the manufacture of well casing characterized by superior resistance to hydrogen sulfide stress corrosion and high yield strength.
Considerable work has been done in recent years to develop higher strength casing steels which exhibit better resistance to failure under stress and corrosive conditions resulting from exposure to liquids containing hydrogen sulfide, as in sour oil applications. The need for higher strength hydrogen sulfide cracking resistant steels has become more apparent with the increasing energy demands and the descrease of easily obtained sweet oil reserves. Oil fields now being explored require drilling to depths beyond 20,000 feet with bottom hole pressures and temperatures exceeding 24,000 psi and 400.degree. F., where hydrogen sulfide is often found in the crude oil. Under these conditions, steel well casing is progressively embrittled in the presence of the hydrogen sulfide and subsequently cracks and fails under the stresses to which the casing is subjected.
Many metallurgical factors influence the sulfide stress cracking behavior of steel well casing. These factors include the microstructure, composition, heat treatment, strength and hardness of the steel. All of these factors are interrelated and must be closely controlled. Small deviations from optimum limits of only one factor, such as the temperatures of heat treatment, will adversely affect sulfide cracking resistance even though other factors such as composition remain unchanged. Chemical composition affects the resistance to hydrogen sulfide cracking of steels by changing such metallurgical characteristics of the steel as hardenability, transformation characteristics and tempering response which, in turn, result in changes in strength and microstructure. The influence of particular alloying and impurity elements on sulfide stress cracking resistance changes from one alloy system to another and the effects of these elements also dramatically change with changes in strength level. As a consequence, the effect on hydrogen sulfide stress cracking resistance of an element in one alloying system cannot be compared to or predicated from the effect of that element in another system.
Prior to the present invention, it had been generally concluded that casing steels having high yield strength levels of about 90,000 psi or higher were generally more susceptible to hydrogen sulfide stress cracking than lower strength steels. Although investigators have recognized the need for higher strength casing steels, the prior art has not provided a steel composition and a compatible heat treatment procedure that made it possible to appreciably increase the yield strength above 90,000 psi and at the same time improve the resistance to hydrogen sulfide stress cracking.
One prior art suggestion for improving the hydrogen sulfide stress corrosion resistance of casing steel is disclosed in U.S. Pat. No. 2,895,861. It is proposed in that patent to use low alloy steels containing chromium, molybdenum, vanadium, silicon and manganese and to subject these steels to a heat treatment consisting of austenitization at an elevated temperature in the range from 1787.degree. to 2012.degree. F., cooling at a speed at least equal to air cooling, and tempering at a temperature in the range of from 1337.degree. to 1472.degree. F. U.S. Pat. No. 2,895,861 specifies that the yield point of the steel should not be greater than 65 kg./mm..sup.2 or about 92,500 psi. The patent teaches that tempering temperatures below 1337.degree. F. (725.degree. C.) and yield strengths higher than 65 kg./mm..sup.2 (92,500 psi) are to be avoided in the case of the specifically disclosed steels which are austenitized and cooled in the manner described because of the detrimental effect on resistance to sulfide stress cracking.
Another hydrogen sulfide stress corrosion resistant steel is disclosed in U.S. Pat. No. 2,825,669. The low alloy steel of that patent contains as essential ingredients, in addition to an extremely close tolerance of carbon, small amounts of manganese, chromium, aluminum and silicon. An aluminum content of from 0.15 to 1.20% by weight is disclosed as being required to promote and accelerate migration and dispersions of carbides into the ferrite grains, impart stress corrosion resistance, and to strengthen ferrite. The composition also may include as nonessential or optional ingredients molybdenum, vanadium and titanium. The steel is given a preliminary anneal by soaking at about 1364.degree.-1436.degree. F. for the apparent purpose of diffusing or dispersing the carbide aggregates throughout the ferrite grains before the carbides are dissolved by a subsequent high temperature austenitizing treatment. According to the patent disclosure, the steel may be used in the state obtained after the dispersion or diffusion treatment, or the steel may be subjected to an optional austenitization, quench and temper treatment. The steel is austenitized at a high temperature in the range of from 1778.degree.-1976.degree. F. The quenching treatment to which the steel is subjected after austenitization may result in a microstructure containing martensite as well as other transformation products such as bainite, etc. As in the case of U.S. Pat. No. 2,895,861, the examples of U.S. Pat. No. 2,825,669 involve steels heat-treated to produce yield strengths less than about 90,000 psi.